Mi madre dejó las plantas adentro porque hacía mucho viento en el balcón.

Because dejó is the preterite, which is usually used for a completed action in the past.

Here, the sentence presents your mother’s action as a single finished event:

• Mi madre dejó las plantas adentro = My mother brought/left the plants inside

By contrast, dejaba (imperfect) would suggest something more habitual, repeated, or backgrounded, such as:

• Mi madre dejaba las plantas adentro cuando hacía viento = My mother used to leave the plants inside when it was windy

So in this sentence:

• dejó = the main completed action
• hacía = the background weather condition

That contrast is very common in Spanish.

Hacía is the imperfect, and it is very often used for weather, background conditions, and descriptions in the past.

In this sentence, the wind is the situation that explains why your mother acted:

• porque hacía mucho viento = because it was very windy

Spanish often uses the imperfect for this kind of background information:

• hacía frío
• hacía calor
• hacía sol
• hacía mucho viento

You could sometimes see hizo mucho viento, but that would usually describe the weather as a completed event or a fact about a period of time, not as the background scene in a sentence like this.

Spanish often uses hacer in weather expressions where English uses to be or another structure.

Examples:

• Hace frío = It’s cold
• Hace calor = It’s hot
• Hace viento = It’s windy
• Hacía mucho viento = It was very windy

So although hacer normally means to do or to make, in weather expressions it works differently and should be learned as a set phrase.

Because mucho modifies a noun, while muy modifies an adjective or adverb.

Here, viento is a noun:

• mucho viento = a lot of wind

Compare:

• mucho viento = a lot of wind
• muy ventoso = very windy

So:

• hacía mucho viento is correct
• hacía muy viento is not correct

In Spanish, weather expressions with hacer often use the noun without an article:

• Hace sol
• Hace frío
• Hace calor
• Hace viento

So hacía mucho viento is the natural structure.

If you said hacía un viento..., that would sound more specific or more literary, often followed by extra description, for example:

• Hacía un viento terrible

But the neutral, everyday form is hacía mucho viento.

Yes. In this sentence, adentro means inside.

• dejó las plantas adentro = left the plants inside

You can also often say dentro, and in many contexts both are possible:

• dejó las plantas dentro
• dejó las plantas adentro

In everyday speech, both are understood. Depending on region and style, one may sound a bit more natural than the other in certain contexts. In Spain, dentro is often more common in neutral usage, but adentro is still perfectly understandable and natural.

Also compare:

• dentro de la casa = inside the house
• adentro = inside, indoors

Spanish often uses the definite article more than English does.

Here, las plantas refers to a specific set of plants already known in the situation:

• Mi madre dejó las plantas adentro = My mother left the plants inside

In English, we also usually say the plants here, not just plants, because we mean specific plants.

So las is not unusual at all; it is the normal article for plural feminine nouns.

Because en means in/on/at, depending on context, and for locations like el balcón, Spanish naturally uses en:

• en el balcón = on the balcony

Even though English says on the balcony, Spanish uses en.

This is very common:

• en la mesa = on the table
• en la pared = on the wall
• en la playa = at the beach
• en el balcón = on the balcony

So you should not translate on word-for-word as a separate Spanish preposition here.

It can suggest either idea depending on context, but in this sentence the natural interpretation is:

• she left/kept the plants inside

The verb dejar basically means to leave. So literally:

• dejó las plantas adentro = she left the plants inside

However, in context, if the plants are normally on the balcony, English might naturally translate the whole idea as:

• My mother brought the plants inside because it was very windy on the balcony

So the exact English wording can vary, even though the Spanish uses dejar.

Because porque (one word, no accent) means because and introduces a reason.

• porque hacía mucho viento = because it was very windy

Compare the different forms:

• porque = because
• por qué = why
• el porqué = the reason
• por que = much less common, used in specific structures

So in your sentence, porque is correct because it explains the reason for your mother’s action.

Because Spanish usually does not need subject pronouns when the verb ending already shows who the subject is.

• dejó already tells you it is he/she/you formal left
• The noun Mi madre makes the subject completely clear

So:

• Mi madre dejó las plantas adentro is natural
• Mi madre ella dejó... would be wrong

You could say Ella dejó las plantas adentro if the subject had already been established and you wanted to say she instead of repeating mi madre, but here it is unnecessary.

There are two main clues:

1. Word order
   Spanish usually places the subject before the verb in neutral sentences:

   • Mi madre
     • dejó
       • las plantas adentro

2. Meaning of the verb
   dejar takes a direct object:

   • someone leaves something somewhere

So here:

• Mi madre = the person doing the action
• las plantas = the thing affected by the action

A helpful breakdown is:

• Mi madre = subject
• dejó = verb
• las plantas = direct object
• adentro = place/result
• porque hacía mucho viento en el balcón = reason

Yes, it is natural and understandable.

A speaker from Spain might also say slightly different versions, such as:

• Mi madre dejó las plantas dentro porque hacía mucho viento en el balcón.
• Mi madre metió las plantas dentro porque hacía mucho viento en el balcón.

The version with metió emphasizes the idea of putting them inside more clearly, while dejó emphasizes left/kept them inside.

So your original sentence is good Spanish, and a learner should understand it without any problem.